When we talk about dimming LED lighting, many articles mix the dimming control and dimming methods, such as combining PWM and other dimming control methods. I plan to explain the dimming method of LEDs and the dimming control methods for LED lighting.
LED's voltage-ampere characteristic and fast switching characteristics determine the dimming methods.The
LED power supply determines the LED lighting's dimming control methods.
An LED (Light-Emitting Diode) is a solid-state semiconductor device that converts electrical energy into visible light. It is composed of P-type semiconductor conductors. When current flows through this chip via a wire, electrons are pushed to the P region. In the N-type semiconductor and the P-type semiconductor, the electrons and holes collide violently in the light-emitting layer, producing photons. Consequently, energy is emitted in the form of photons.
2 types of dimming methods for LED
DC dimming controls the brightness by adjusting the LED lamp's current or the circuit power (power = voltage x current), so changing the voltage or current can change the LED lighting brightness.
An LED is a diode that can emit light. It relies on electrons to flow to the P region, where electrons collide with holes and then emit energy in the form of photons. Therefore, its light-emitting principle determines that the brightness of the LED is proportional to its forward current. Most LEDs have a rated current, but this does not mean that the LED can only operate at these rated currents.
For example, a 1-watt and a 3-watt LED are the same model. If the current increases from 350mA to 700mA, the power increases from 1W to 3W, so that this LED can work at any value between 350mA and 700mA.
Therefore, adjusting the current passing through the LED can change its light intensity.
Volt-ampere characteristics refer to the relationship between the voltage applied to both ends of a conductor and the current flowing through the conductor.
According to the diode volt-ampere characteristics, when the forward voltage is added above the turn-on voltage, a slight voltage fluctuation causes the current flow through the light-emitting diode LED to fluctuate exponentially.
The nonlinearity of the LED volt-ampere characteristics means that a small voltage change will cause a large current change. From the volt-ampere characteristics, a 10% change in the power supply voltage will cause a change in the forward current several times that magnitude.
The LED voltage and current are not fixed. In fact, the volt-ampere characteristics of LEDs are not fixed but vary with temperature.
Assuming that a 3.3V constant voltage source is used to work 20mA at room temperature, and when the temperature rises to 85 degrees, the current will increase to 35-37mA, and its brightness will not increase. The current increase will only increase its temperature, leading to increased light decay and reduced life.
From the LED volt-ampere characteristics, we know that LEDs cannot be dimmed by simply adjusting the voltage and current. Generally, a wide-voltage, constant-current dimming power supply is used. The constant current source means that the power supply determines the current size of the LED, and the wide voltage means that the power supply will automatically match the appropriate power supply voltage according to the LED's voltage-ampere characteristics.
For example, in an LED lamp with an input of 24V, 8pcs 1W high-power LEDs are connected in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V, so the 8 series is 26.4V, so a 26.4V constant current source should be used. When dimming, the current is reduced to 100mA. Currently, the forward voltage is only 2.8V, and the 8 series is 22.4V, so the load voltage should also be lower.
If the buck-type (wide voltage) constant current source is adjusted to a low forward voltage, the LED's load current will also decrease significantly. Hence, the step-down ratio is very large, which may exceed the normal working range of this buck-type (wide voltage) constant current source, making it unable to work and flicker.
Additionally, if the buck-type (wide voltage) constant current source operates at low brightness for an extended period, its efficiency will decrease, and the temperature rise will increase. It will not work because the efficiency of the buck-type (wide voltage) constant current source is related to the step-down ratio. The larger the step-down ratio, the lower the efficiency and the greater the power consumption on the chip, which will damage the life of the constant current source and the LED light source.
PWM dimming achieves the dimming effect by controlling LED lamps' on/off time and duty cycle.An
LED is a diode that can achieve fast switching. Its allowable switching speed can be as high as microseconds, unmatched by any light-emitting device (so traditional lamps cannot be dimmed if they are too slow to switch on and off). Therefore, as long as the power supply is changed to a pulse-constant current source, the brightness can be altered by changing the pulse width. This method is called pulse width modulation (PWM) dimming.
PWM dimming does not adjust the brightness by changing the power but by alternating the screen on and off. When PWM dimming is enabled, it does not emit light continuously, but instead lights up and turns off the screen periodically. When the alternating on and off is fast enough, the naked eye will perceive the LED as always being on.
During the lighting and turning off process, the longer the off state lasts, the lower the brightness of the LED will be to the naked eye. The longer the lighting time, the shorter the off time, and the brighter the LED will be. Most of them are fixed-frequency, and changing the brightness involves adjusting the duty cycle.
- No LED color spectrum offset will be generated because the LED always works between full amplitude current and 0.
- It has exceptionally high dimming accuracy because the pulse waveform can be completely controlled to very high accuracy, so it is easy to achieve an accuracy of one ten-thousandth.
- Even if dimming is performed over an extensive range, flickering will not occur. Since the working conditions of the constant current source (boost ratio or buck ratio) will not be altered, overheating and other issues are even less likely to occur.
- It can be combined with digital (DALI/DSI/DMX 512) control technology for control because the digital control signal can be easily converted into a PWM signal.
Because the LED is fast-switching, if the operating frequency is very low, the human eye will feel flickering. To fully utilize the human eye's visual residual phenomenon, the operating frequency should be higher than 100 Hz, preferably 200 Hz.
The whistling sound caused by dimming is not perceptible to the human ear above 200Hz, but it is within the range of human hearing, extending up to 20kHz. At this time, you may hear a slight hissing sound.
Based on the LED power supply dims LED lighting. The LED power supply can be considered a component of the LED lamp. Just like the lighting and extinguishing of the light requires a switch control, the lamp also needs to be dimmed according to the instructions received from the power supply. The different uses of this instruction form various dimming control methods for the lamp. Let's talk about the six standard LED lighting equipment control methods on the market:
Leading-edge dimming uses a thyristor circuit. The input voltage is chopped, starting from AC phase 0, and is applied only when the thyristor is turned on.
The working principle involves adjusting the conduction angle of each half-wave of the AC to modify the sine waveform, thereby altering the practical value of the AC and achieving the purpose of dimming. Leading-edge phase control dimmers typically utilize thyristors as switching devices, also known as triac dimmers.
Advantages: Low dimming cost, compatible with existing lines, no need for rewiring (other dimming methods require signal lines).
Disadvantages: Many LED triac dimming drivers struggle to fully match dimmers, resulting in an uneven dimming process that cannot be fully turned off and may cause slight flickering.
The trailing edge phase-cut control dimmer is similar to the leading edge phase-cut; it is generally made up of field-effect transistor (FET) or insulated-gate bipolar transistor (IGBT) devices. The trailing edge phase-cut dimmer generally uses MOSFET as the switching device, so it is also called a MOSFET dimmer, commonly known as an "MOS tube."
MOSFET is a fully controlled switch that can be controlled to open or close, so there is no phenomenon that the thyristor dimmer cannot be completely turned off.
Advantage: A MOSFET dimming circuit is more suitable for capacitive load dimming than a thyristor. There is no minimum load requirement, and it can achieve better performance on a single lighting device or a very small load. MOS tubes are rarely used in dimming systems and are generally only made into knob-type single-lamp dimming switches.
Disadvantage: The MOS tube dimming method has not been developed due to its high cost, relatively complex dimming circuit, and difficulty in achieving stability.
Therefore, compared to MOS tube dimming, the triac dimmer still dominates the majority of the dimming system market.
The 1-10V dimming device features two independent circuits: a normal voltage circuit that controls the power to the lighting equipment, and a 1-10V signal circuit that provides a reference voltage to indicate the dimming level to the lighting equipment.
0-10V dimming controllers were previously commonly used for dimming control of fluorescent lamps.
Now, with the addition of a constant power supply to the LED driver module and dedicated control circuits, 0-10V dimmers can also support a wide range of LED lighting lamps.
However, the application's disadvantages are also very obvious. The low-voltage control signal requires additional lines, which significantly increases the construction requirements.
DALI is the "Digital Addressable Lighting Interface" acronym, which means "digital addressable lighting interface". It is an internationally open lighting control communication protocol, IEC62386, which specifies the digital communication control method between electronic ballasts, control units, and various sensors. In other words, DALI is a protocol specifically designed for lighting control, which allows for the control of each lamp individually, enabling adjustments to brightness, color temperature, color, and other settings. Additionally, it supports group control of lamps, allowing for the creation of different scenes.
In the 1990s, Europe began to develop and research digital fluorescent lighting control systems. Major European electronic ballast manufacturers (such as Halvar, Hüco, Philips, Osram, Tridonic, Trilux, etc.) have joined the formulation of DALI standards. The DALI definition standard aims to establish a simple lighting system with a clear and structured approach.
It is used for intelligent and high-performance lighting management in indoor areas, and the controlled object is the ballast. It can be interfaced with a building management system (BMS), such as the KNX system. DALI is typically used as a small component of a building's electrical control system. For example, it can be integrated into the KNX system through the KNX-DALI gateway to achieve a wide range of lighting control.
The DALI standard defines a DALI network as including a maximum of 64 units (independently addressable), 16 groups, and 16 scenes. Different lighting units on the DALI bus can be flexibly grouped to achieve various scene control and management options. In a DALI network, 30 to 40 control instructions can be processed per second.
DALI is not a true point-to-point network. It replaces the 1-10V voltage interface to control the ballast. Compared with the traditional 1-10V dimming, the advantage of DALI is that each node has a unique address code and has feedback. The longer distance dimming will not have signal attenuation like 1-10V, but in engineering practice, this distance should not exceed 200 meters.
A DALI network can only control 21 (64/3 colors) full-color LED lamps. DALI is oriented to traditional lighting control, focusing on the system's static control, reliability, stability, and compatibility. The scale of an LED lighting system is significantly larger than that of a DALI system, which necessitates a connection to a larger bus network. Therefore, the DALI system is often incorporated into other bus systems as a subsystem in large lighting projects.
- Digital dimming, precise, stable, and smooth dimming.
- DALI can communicate bidirectionally and provide feedback on the status of lamps to the system.
- Single lamp control is more flexible.
- DALI has strong anti-interference ability.
- Complex signal line layout.
- High price.
- The DALI dimming driver still needs to consume standby power when turning off the lights to ensure that the microcontroller is always on standby.
DMX (Digital Multiplex) means multi-channel digital transmission.
The DMX512 control protocol is an industrial standard for data transmission between lighting controllers and equipment released by the American Stage Lighting Association (USITT) in 1990. It covers electrical characteristics, data protocols, data formats, and other aspects.
A DMX512 interface can connect up to 512 channels, indicating that this device is a digital transmission and dimming device with 512 dimming channels. It is an integrated circuit chip that separates control signals, such as brightness, contrast, and chromaticity, processes them separately, and adjusts the analog output level value by controlling the digital potentiometer to regulate the brightness and hue of the video signal. It divides the light level from 0 to 100% into 256 levels. The control system can achieve full color by utilizing R, G, B, and 256 grayscale levels.
Its output signal transmission method is RS-485 differential signal transmission, which effectively guarantees the reliability of its signal. The ideal transmission distance can reach 200m. However, in actual applications, due to signal interference and attenuation, adding a repeater to strengthen the signal is generally recommended at about 40 meters.
Although the dimming principle may seem complicated, the DMX512 system can be summarized into three main components: DMX master control, DMX decoder, and LED lamps.
Compared to the traditional analog dimming system, the digital lighting system based on the DMX512 control protocol has brought significant changes to architectural lighting, nighttime lighting, and the lighting effects of large and medium-sized film and television studios, as well as various shows, thanks to its powerful control function.
However, in actual applications, multiple controllers are interconnected to control complex lighting solutions, and the operating software design will be more complicated. The controller has many wiring, and the construction is relatively cumbersome, which results in a high cost for later maintenance.
We can see that LED lighting dimming is controlled by the lamp's power supply. The main function of the dimming system is to give a command signal to the power supply. In addition to triac dimming, a special signal line must be set up to transmit the signal, which is inconvenient.
So, can wireless networks be used to transmit dimming signals? The answer is yes, and they are being used increasingly. The most used is the Bluetooth Mesh solution.
The Bluetooth Mesh network is a new Bluetooth Low Energy (BLE) network topology for establishing many-to-many (many-to-many) device communications. It enables you to establish an extensive network comprising multiple devices. The network can contain dozens, hundreds, or even thousands of Bluetooth Mesh devices, which can communicate with each other.
Bluetooth Mesh has five significant advantages.
- The most significant advantage is that Bluetooth Mesh can communicate directly with smart terminals without a gateway, which can fully utilize the benefits of a good human-computer interaction experience.
- There can be no gateway that is low-cost, or a gateway can be set up to provide more network control methods.
- Very convenient installation and the user experience is particularly good;
- Strong low-power support capability.
- Strong networking capability: 250 nodes can be grouped in two directions, and 1,000 can be grouped in one direction.
The smart lighting system is an ideal indoor setting for a Bluetooth mesh network application. The lighting system's layout range is generally very close, and the mutual broadcast between Bluetooth modules can be detected. This is superior to the ZigBee mode, which requires a gateway for on-site interoperability.
With Bluetooth Mesh, you can control all functions in your smart home system simultaneously, easily, and efficiently using just one control device. The robust architecture of Bluetooth Mesh can also be expanded to meet the needs of offices, factories, industrial environments, and even cities, connecting millions of nodes. The disadvantage is that the lamps cannot be physically started if the system fails.
RF or "Radio Frequency" dimming provides a way to adjust a strip's brightness remotely. Unlike wired plug-in LED dimmers, LED strips can be dimmed remotely at distances of up to 20 meters in diameter. This is generally used for manual control or as a backup for automatic, wireless control (when Bluetooth or Wi-Fi lighting control systems fail).
WiFi networked smart dimmers allow you to control LEDs remotely via a WiFi connection. When setting up a "smart" office lighting system, these dimmers can be controlled through an online system. The disadvantage of this method is that it requires a good network distribution in the lighting area; that is, the price of smart dimming controllers is slightly expensive, and a dedicated system needs to be developed to control lighting dimming or access cloud platform management.
ZigBee dimming is usually implemented by a ZigBee device (such as a ZigBee light bulb), receiving commands sent from a controlling device (such as a mobile phone application or other ZigBee controller). Dimming typically involves sending a PWM (pulse-width modulation) signal to adjust the voltage or current supplied to the light bulb.
Here is a simple example of ZigBee dimming implementation:
- The device starts up and enters the standby state.
- The controller sends a dimming command, including a brightness value (for example, 25%).
- The receiving device converts the brightness value into a PWM period and duty cycle.
- The device starts sending a PWM signal to change the brightness.
Professional knowledge can be somewhat complex. In actual applications, experienced designers can design customized dimming solutions tailored to specific conditions. If you are looking for a dimmable LED downlights factory, please don't hesitate to contact Top Sun Lighting to discuss this further.
